Kofron et al U.S. Ser. No. 429,407, filed Sept. 30, 1982, titled SENSITIZED HIGH ASPECT RATIO SILVER HALIDE EMULSIONS AND PHOTOGRAPHIC ELEMENTS, commonly assigned, discloses multicolor photographic elements in which at least one of the blue, green, and red recording emulsion layers is comprised of a dispersing medium and silver halide grains, wherein at least 50 percent of the total projected area of the silver halide grains is provided by chemically and spectrally sensitized tabular silver halide grains having a thickness of less than 0.3 micron, a diameter of at least 0.6 micron, and an average aspect ratio of greater than 8:1. Kofron et al specifically discloses the use of high aspect ratio tabular grain emulsions in which the tabular grains are comprised of silver bromoiodide (iodide being limited by its solubility in silver bromide to about 40 mole percent), silver bromide, silver chloride, silver chloride containing minor amounts of bromide and/or iodide, and silver chlorobromide. (Except as otherwise indicated, all references to halide percentages are based on silver present in the corresponding emulsion, grain, or grain region being discussed; e.g., a grain consisting of silver bromoiodide containing 40 mole percent iodide also contains 60 mole percent bromide.) Kofron et al contains no disclosure of high aspect ratio tabular grain silver iodide emulsions, and, because of the rarity with which silver iodide emulsions are employed in multicolor photographic elements, bases its teachings on the properties of the silver halides more commonly employed in multicolor photography. For example, Kofron et al teaches increasing the permissible maximum thickness of the tabular grains from 0.3 micron to 0.5 micron to increase blue light absorption, recognizing that the thicker tabular grains are better able to assist the blue spectral sensitizing dyes in absorbing blue light. Further, Kofron et al discusses multicolor photographic elements in which high aspect ratio tabular grain blue recording emulsion layers overlie minus blue (green and/or red) recording emulsion layers and discusses the effects of blue light reaching these minus blue recording emulsion layers. Jones and Hill U.S. Ser. No. 430,092, filed Sept. 30, 1982, titled PHOTOGRAPHIC IMAGE TRANSFER FILM UNIT, commonly assigned, is essentially cumulative in its teachings, but is directed specifically to image transfer film units. Maskasky U.S. Ser. No. 431,855, filed Sept. 30, 1982, titled CONTROLLED SITE EPITAXIAL SENSITIZATION, commonly assigned is essentially cumulative in its teachings, but is directed specifically to the sensitization of high aspect ratio tabular grains by silver salt epitaxy.
Radiation-sensitive silver iodide emulsions, though infrequently employed in photography, are known in the art. Silver halide emulsions which employ grains containing silver iodide as a separate and distinct phase are illustrated by Steigmann German Pat. No. 505,012, issued Aug. 12, 1930; Steigmann, Photographische Industrie, "Green- and Brown-Developing Emulsions", Vol. 34, pp. 764, 766, and 872, published July 8 and August 5, 1938; Maskasky U.S. Pat. Nos. 4,094,684 and 4,142,900; and Koitabashi et al U.K. patent application No. 2,063,499A. Maskasky Research Disclosure, Vol. 18153, May 1979, Item 18153, reports silver iodide phosphate photophic emulsions in which silver is coprecipitated with iodide and phosphate. A separate silver iodide phase is not reported.
The crystal structure of silver iodide has been studied by crystallographers, particularly by those interested in photography. As illustrated by Byerley and Hirsch, "Dispersions of Metastable High Temperature Cubic Silver Iodide", Journal of Photographic Science, Vol. 18, 1970, pp. 53-59, it is generally recognized that silver iodide is capable of existing in three different crystal forms. The most commonly encountered form of silver iodide crystals is the hexagonal wurtzite type, designated .beta. phase silver iodide. Silver iodide is also stable at room temperature in its face centered cubic crystalline form, designated .gamma. phase silver iodide. A third form of crystalline silver iodide, stable only at temperatures above about 147.degree. C., is the body centered cubic form, designated .alpha. phase silver iodide. The .beta. phase is the most stable form of silver iodide.
James, The Theory of the Photographic Process, 4th Ed., Macmillan, 1977, pp. 1 and 2, contains the following summary of the knowledge of the art:
According to the conclusions of Kokmeijer and Van Hengel, which have been widely accepted, more nearly cubic AgI is precipitated when silver ions are in excess and more nearly hexagonal AgI when iodide ions are in excess. More recent measurements indicate that the presence or absence of gelatin and the rate of addition of the reactants have pronounced effects on the amounts of cubic and hexagonal AgI. Entirely hexagonal material was produced only when gelatin was present and the solutions were added slowly without an excess of either Ag or .sup.+ I.sup.-. No condition was found where only cubic material was observed.
Tabular silver iodide crystals have been observed. Preparations with an excess of iodide ions, producing hexagonal crystal structures of predominantly .beta. phase silver iodide are reported by Ozaki and Hachisu, "Photophoresis and Photoagglomeration of Plate-like Silver Iodide Particles", Science of Light, Vol. 19, No. 2, 1970, pp. 59-71, and Zharkov, Dobroserdova, and Panfilova, "Crystallization of Silver Halides in Photographic Emulsions IV. Study by Electron Microscopy of Silver Iodide Emulsions", Zh. Nauch. Prikl. Fot. Kine, March-April, 1957, 2, pp. 102-105.
Daubendiek, "AgI Precipitations: Effects of pAg on Crystal Growth(PB), III-23", Papers from the 1978 International Congress of Photographic Science, Rochester, New York, pp. 140-143, 1978, reports the formation of tabular silver iodide grains during double-jet precipitations at a pAg of 1.5. Because of the excess of silver ions during precipitation, it is believed that these tabular grains were of face centered cubic crystal structure. However, the average aspect ratio of the grains was low, being estimated at substantially less than 5:1.
Maskasky U.S. Ser. No. 451,309 filed Dec. 20, 1982, and commonly assigned, titled GAMMA PHASE SILVER IODIDE EMULSIONS, PHOTOGRAPHIC ELEMENTS CONTAINING THESE EMULSIONS, AND PROCESSES FOR THEIR USE, discloses the first high aspect ratio tabular grain silver iodide emulsions in which the grains are of a face centered cubic crystal structure, as is characteristic of silver iodide.